This invention relates to biological implants for human subjects. More particularly, this invention relates to a synthetic intervertebral disc that is implantable to replace a damaged intervertebral disc.
Human intervertebral discs act as cushions or shock absorbers between vertebrae. The discs include a fibrous outer layer surrounding a gel-like matrix. The fibrous outer layer contains the gel-like matrix which thereby provides the cushioning effect. As much as 500 lbs. of pressure is exerted on a single intervertebral disc when the body is in certain positions. In a single day, a natural disc may compress and extend over 13,000 times. Natural discs often deteriorate with age or are otherwise injured, causing nerves to be pinched, pain, and subsequent deterioration of the vertebral surfaces. The body continually nourishes the natural discs via nutrients from the fluid that bathes nearby tissue, but this nourishment will not repair natural discs that are significantly damaged.
When the natural discs are damaged, the physical condition created often greatly affects the mobility of the individual. Any motion of the torso causes the position of the spine to change, thereby altering the stresses on each vertebrae. The damaged disc prevents the vertebral bones from moving in natural positions and usually places pressure on surrounding tissue. Resultantly, the spinal column and the nerve roots surrounding the affected vertebral disc are often pressured by the damaged disc and the individual is usually in great pain. In some cases, the damaged disc places continual pressure on the spinal column thereby causing continuous pain.
The oldest approach to repairing severely damaged intervertebral discs, included removing the damaged natural disc and fusing the two adjacent vertebral bones into one piece. Typically, fusing the vertebral bones was performed by grafting bone between the adjacent vertebrae using metal plates and screws to hold the graft in place until it healed. Once healed, the spinal fusion prevented the vertebral bodies from moving relative to one another and relieved the pressure formerly exerted by the damaged disc on the nerve roots. However, because the procedure prevented movement that usually occurred in that section of the spinal column, patients often then suffered from painful strain on muscles, ligaments, and other tissues that surrounded the fusion. Lack of movement in the spine also put additional pressure on the discs above and below the fusion, sometimes damaging the adjacent natural discs.
Certain prosthetic structures were introduced as vertebral disc implants. Various types of polyolefin rubber, polyethylene, and silicone composites were used to create synthetic discs that mimicked the shape and structure of the natural intervertebral discs being replaced. These materials were produced to match the strength and tensile properties of a human intervertebral disc. However, with the materials currently available, the resulting synthetic discs were not durable enough to withstand the intervertebral forces and have failed soon after being implanted.
U.S. Pat. No. 4,750,769 to Hedman et. al. discloses a synthetic disc having upper and lower plates hinged together at an anterior location that are springed against each other to cause separation of the plates. However, the construction of the synthetic disc had several limitations. Most importantly, the Hedman et al. disc employed a hinge mechanism that allowed compression between adjacent vertebrae to occur only anteriorly. Such a limitation on motion allowed necessary forward flexion but did not allow posterior and lateral flexion or extension and did not allow twisting of the vertebral column at the site of the implant. Therefore, the device does not allow natural motion between the adjacent vertebrae. Further, because the implant included moving parts that were continually exposed to deposition from body tissues, their movement was limited over time and could damage adjacent body tissue. Further, because of the construction of the device, the vertebrae above and below the disc had to be cut-down to make room for the device, further traumatizing the body.
U.S. Pat. No. 4,309,777 to A. A. Patil describes a synthetic disc composed of two cups, one overlapping the other, that are held apart by springs. With this device, the cups could only move in a single dimension with respect to one another. Therefore, the springs acted only as shock absorbers between the cups. The design did not facilitate flexion of the spine in any direction. Further, because one cup slid over the other cup as the springs were compressed and because there was no lubrication between the cups, the movement between the cups was substantially inhibited by the friction between the cups. Further, body tissues such as nerves, ligaments, and muscles could become caught in the moving parts over time, damaging the tissues, and likely rendering the device incapable of movement.